Sugoroku is Yugi's grandfather. Ever since Yugi was little, he's been taking care of him as his guardian. Sugoroku was an archeologist and made many trips to Egypt. He and his friend were the first ones in the scientific community that really noticed that the duel monsters card game had connections to ancient Egypt. He is also the one that gave Yugi the pieces of the mysterious Millennium puzzle.
Sugoroku is an expert duelist and taught his grandson all that he knew about the card game. He even gave Yugi is own deck, which Yugi uses in almost every battle. Sugoroku also taught Jounouchi how to play the card game. He believes in the heart of the cards and teaches these principles to those he cares about.